Rubber toughened acrylic resins are widely used in applications where the beneficial properties of acrylics (clarity, weathering, etc.) are desired, but where standard unmodified acrylic resins lack the desired level of impact toughness. The usual way of toughening an acrylic resin is by incorporating impact modifier into the acrylic matrix.
Large particle size impact modifiers (>220 nm volume average particle size) generally provide good impact performance but lead to poor appearance, such as surface haze upon thermoforming (low gloss). Small particle size modifiers (<220 nm volume average particle size) provide good appearance after melt processing but do not consistently provide good impact performance.
Rubber particles with bimodal particle size distribution have been used to achieve a balance of high gloss, impact strength and stiffness in polystyrene and polypropylene polymers as described in U.S. Pat. Nos. 5,985,997; 6,323,282; and 6,6660,808. These references describe the use of bimodal diene copolymer rubbers particles. There is no mention of acrylic impact modifiers in an acrylic-based matrix material.
The blending of particle populations with different mean particle size and therefore different properties often produces a product with physical propereties that are the average—or often the worst of those of each set of particles.
Surprisingly it was found that an acrylic composition having impact modifiers with an appropriate bimodal particle size distribution provides an optimal balance of impact performance and appearance after melt processing. By replacing an amount of the small particle size impact modifiers with large impact modifiers such that the fraction of large impact modifier in the resin is maintained below 20%; both good impact and good appearance as a part is formed, and after thermoforming can be achieved. The use of impact modifiers with a bimodal particle size distribution provides a unique balance of properties that can not be achieved with either modifier alone or any mono particle size approach.